Dreaming of Martians

The human imagination conceives of Mars in many different ways. One can see the
bright red dot of light in the night sky; if you watched the lunar eclipse last
week you probably also noticed Mars, near the Moon. Then there is the planet of
our science fiction stories, whose surface features were once thought to be
covered with canals and fantasized Martians. Finally, there is the planet
itself that is slowly being revealed to us by a fleet of spacecraft and
landers. Sometimes it is hard to recall that all three conceptions are
referring to the same place.

Roughly every two years (778 days, to be precise) Mars and Earth are aligned in
their orbits around the Sun. At that time, Mars is closest to us and appears at
its brightest. Furthermore, the orbit of Mars is not a perfect circle but an
ellipse; when this alignment occurs at the same time that Mars happens to be in
the part of its orbit that comes closest to the Sun (and thus to Earth) Mars looks
especially bright. This happens in intervals of about 15 or 17 years. The
summer of 2018 marks one of those favorable approaches.

At such times, amateurs with small telescopes look eagerly to make out its
bright caps of polar ice, or strain to spot dark features along the equator of
Mars. It was during such favorable observing times in the late 1850s that Fr.
Angelo Secchi, the Jesuit astronomer at the Roman College (who celebrates his
bicentennial this year), first noted dark surface streaks he labeled as
“canali.” Later that century, Giovanni Schiaparelli, observing from Milan,
thought he detected narrow lines connecting the darker regions; he also used
the word “canali” to describe them. When the American astronomer Percival
Lowell also thought he saw those lines, he misunderstood them to be artificial
canals, perhaps dug by desperate Martians trying to fetch water from those
polar ice caps. Three years after Lowell’s first publication, H. G. Wells had
turned this idea into his science fiction classic The War of the Worlds. Alas,
both Lowell and Schiaparelli were fooled by an optical illusion; as our
spacecraft have shown, the dark regions seen by Secchi are real but the
artificial canals are not.

So far this season our Mars viewing has been blocked by bad weather — on Mars,
not Earth. Spacecraft orbiting Mars tell us that a large dust storm has been
obscuring its surface since mid June, though there are signs that it may be
slowly abating.

Meanwhile, however, recent reports of an experiment on ESA’s Mars Express
orbiter has revived the idea of liquid water and indeed possibly life on Mars.
Again, following the footsteps of Schiaparelli and Secchi, it is a team of
Italian astronomers led by Dr. Roberto Orosei from the University of Bologna
who have made this discovery.

Their experiment, the Mars Advanced Radar for Subsurface and Ionosphere
Sounding (MARSIS), sends radar waves to penetrate and map out the structure of
the upper layers of the Martian surface. Probing the area beneath the southern
pole cap (which consists both of water and carbon dioxide ice) they mostly
found volcanic rock, as expected. But a thin flat region about twenty
kilometers across and 1500 meters beneath the surface reflected back the radar
waves very strongly, just as has been seen by liquid water layers on Earth.

The surprise is not that there is water on Mars; we can see dried-up river beds
on its surface, and we have found ice beneath a layer of sand near the poles.
But today the Martian surface is too cold and dry to support flowing rivers.
The real news it that the water seen by MARSIS is actually still liquid, not
frozen. But ice cannot reflect the radar waves like MARSIS has seen. Instead,
one needs liquid water… in particular, a briny liquid filled with dissolved
salts.

The region of this layer under the Martian pole is thought to be very cold,
perhaps as much as -60 C. Ordinary sea salt (sodium chlorine) would not be able
to keep the water liquid at that temperature, but perchlorates (made of
chlorine and oxygen atoms) can do so. Alas for anyone dreaming of Martian
microbes, perchlorate brines are not particularly friendly places for life to
survive. Still, we have already found some life forms on Earth that can live in
extreme conditions; it is not out of the question that Mars could host a form
of bacteria able to survive in such a brine.

Supporting this idea of life on Mars is another recent research paper by a team
of scientists led by Dr. Jennifer Eigenbrode, a biogeochemist at NASA’s Goddard
Space Flight Center, using instruments on NASA’s surface rover, Curiosity. This
rover has a device, Sample Analysis for Mars (SAM) to look for organic
chemicals — complex carbon compounds that may, or may not, be formed by
biological processes — in Martian samples collected at various places on the
surface of the planet. One site they chose is rich in mudstones formed back
three billion years or so ago, when Mars was warmer and wetter. As the SAM
instrument heated up the samples, fragments of complex organic chemicals were
broken off and measured. They look like the sorts of fragments that come from
“kerogens”, complex chains of hydrocarbon rings like those made by living
creatures… but also found in meteorites and igneous lavas which are also seen
on Mars, formed in conditions where life could not have existed.

Is there life on Mars? There are certainly no canals, nor any canal-digging
Martians. But we now know that there is liquid water and we have evidence that
may not prove there’s life but is certainly consistent with some kinds of life
forms. It is interesting that the samples with the organic materials can be
dated to the time when life was just getting started on planet Earth. It would
certainly be interesting to see how life may have developed on another planet.
Or, if it is finally determined that life did not develop on Mars, it would be
good to know what stopped it from happening there.

The scientific work continues. The
Mars 2020 lander is planning to gather together samples for yet another lander,
later that decade, which can return them to our laboratories on Earth.
Meanwhile, anyone with a clear sky this month can go outside and see the red
planet for themselves… and dream of Martians, large or small.